Question

If $100N$ force is applied on the block. Then, frictional force acting on the block will be (in Newton),

Answer 1

Answer: 100

As we know that the coefficient of static friction determines if the relative slipping between the block and the surface is happening or not.
Here, the reaction force,
$N=mg=30\times 9.8=294N$ .
So, the static frictional force,
$f_s={\mu }_{s}N=0.4\times 294=117.6N$
which is greater than applied force. So, no motion will occur and the maximum frictional force applied will be the same as the applied force. So, it is $100N$ .